Generally, with respect to a reaction which cannot proceed without very high energy, for advancing such a reaction with extremely low energy, there is used a photocatalyst which causes an electron excitation state by irradiation with light.
As photocatalysts, there have been known so-called semiconductor catalysts, such as titanium oxide, zinc oxide, cadmium sulfide, and tungstic oxide, and metal complex catalysts, such as a ruthenium bipyridyl complex.
Among these photocatalysts, titanium oxide (TiO2) is the most stable and substantially not biologically poisonous, and therefore is practically used as the photocatalyst for decomposing and removing nitrogen oxides and organic substances in air. However, for titanium oxide only the ultraviolet light having a wavelength of 380 nm or less can be used. The ultraviolet light in this region of wavelength is as small as 4% of the sunlight, and therefore titanium oxide achieves only the utilization efficiency for sunlight, which is the most abundant light source, 4% at the most, and practically at the most 1% thereof.
Typical examples of photocatalytic reactions of titanium oxide include reduction of O2 (formation of H2O2), oxidation of H2O (generation of O2), reduction of methylbiologen, reduction of N2, drainage treatment, and a photocatalytic Kolbe reaction {CH3COOH→CH4(gas)+CO2}.
Titanium oxide has a photocatalytic ability as well as a photoelectrode ability to electrolyze water, which is known as a Honda-Fujishima effect, and a photovoltaic ability used in a solar cell.
JP-A-2004-290748 (Japanese Patent No. 4214221) and JP-A-2004-290747 (Japanese Patent No. 4247780) show fused quartz treated with a halogen acid as a material having a photocatalytic ability similar to that of titanium oxide.
International Publication No. WO2005/089941 shows synthetic quartz treated with hydrofluoric acid as a material having a photocatalytic ability.
This photocatalyst functions as a photocatalyst in a wavelength region of 200 to 800 nm, which is wider than the wavelength region for the photocatalyst using the fused quartz shown in JP-A-2004-290748 and JP-A-2004-290747.
With respect to the synthetic quartz treated with hydrofluoric acid, International Publication No. WO2005/089941 has, at paragraphs [0021] to [0023], the following description.
The synthetic quartz is activated by the treatment with hydrogen fluoride as mentioned above is explained as follows. When SiO2 and HF are in contact with each other, Si on the surface is bonded to F, so that the bonded electrons are drawn toward the F side and the back bond is weakened. As a result, this site is attacked by the separated H+H− molecules, and the back bond is cleaved, so that Si on the uppermost surface is fluorinated, and simultaneously one of the bonds in the layer immediately below the surface is hydrogenated.
The above state is successively transmitted, and Si on the uppermost surface is finally separated in the form of SiF4, so that SiH3 radicals remain on the surface.
In the SiH3 radicals, however, the Si—Si bond between Si in the radical and Si in the next layer is very weak, and further the bonded electrons are weakly drawn toward the H side, and therefore the Si—Si bond is easily cleaved, so that Si is easily replaced by H in the HF molecules to give a form of SiH. Therefore, H is exposed on the Si (111) surface, thus causing an activated state.
The synthetic quartz treated with hydrogen fluoride is separated from the solution, and washed with distilled water 2 to 5 times, followed by air drying, to obtain the photocatalyst.
The synthetic quartz is activated by hydrogen fluoride as mentioned above, but natural quartz, which comprises the same crystalline silica, is not activated by hydrogen fluoride. The reason for this has not yet been elucidated.
International Publication No. WO2006/095916 shows zinc oxide, tin dioxide, tungsten oxide, and silicon carbide as semiconductor photoelectrode materials other than titanium dioxide used in the ultraviolet region.
Further, there are shown silicon, gallium arsenide, strontium titanate, cadmium selenide, and gallium phosphide as semiconductor photoelectrode materials used in the visible light region.